A boundary microphone is also called a surface mount microphone because it is used by being placed on a table or a floor surface in a TV studio, a conference room, or the like. One example of the boundary microphone is shown in FIG. 5.
A microphone casing 1 for the boundary microphone is made up of two parts: a flat base part 10 the upper surface side of which is open, and a microphone cover 20 having many openings (sound wave introducing holes) that is attached to the base part 10 so as to cover the upper surface of the base part 10.
Usually, the base part 10 is formed by casting such as zinc die casting, and as the microphone cover 20, a punched plate (perforated plate) is used. In place of the punched plate, a wire mesh body is sometimes used.
The microphone cover 20 is screw-mounted to the base part 10. Screw-mounting at many points impairs the appearance of microphone; therefore, in many cases, the microphone cover 20 is screw-mounted to the base part 10 at one point.
That is, substantially in the center of the microphone cover 20, a screw insertion hole 21 is formed, and on the base part 10 side, a boss 11 having an internally threaded hole 12 is erectingly provided. By threadedly engaging an externally threaded screw 22 for fixation with the internally threaded hole 12 in the mating boss 11 through the screw insertion hole 21, the microphone cover 20 is fixed to the base part 10.
Thereby, a shielded space is formed in the microphone casing 1, and therein, a microphone unit 31 is housed in a state of being mounted on a circuit board 30. As the microphone unit 31, a condenser microphone unit is usually used.
On the circuit board 30, an impedance converter, a tone control circuit, an output circuit, and the like (all of these are not shown) are mounted. The circuit board 30 is connected with a microphone cord 32, and the microphone cord 32 is pulled out of the base part 10 via a cord bush 33.
The punched plate used for the microphone cover 20 is manufactured as follows: many holes are formed in, for example, a steel sheet, the steel sheet formed with many holes is cut into a predetermined size, and the cut steel sheet is fabricated into a target shape with a press. Since the punched plate is a plate formed with many holes, a peripheral edge part 20a (cutting face) of the microphone cover 20, which is in contact with the base part 10, is uneven and serrated.
In the case where the base part 10 is formed by die casting, the surface thereof is not smooth, so that the electrical connection between the base part 10 and the microphone cover 20 is made by point contact at many points. The interference caused by ordinary electromagnetic waves such as VHF or UHF band generally used at broadcasting stations and the like can be precluded sufficiently by the shield provided by the base part 10 and the microphone cover 20.
In recent years, countermeasures against the generation of noise caused by cellular phones that have come into wide use rapidly have been needed urgently. The use of a cellular phone produces considerably strong electromagnetic waves (for example, in the range of several centimeters to several tens centimeters, the field intensity reaches several ten thousands times the field intensity produced in the city by commercial electric waves). Therefore, the shield provided by the above-described point contact between the base part 10 and the microphone cover 20 is insufficient: if a cellular phone is used near a microphone, noise may be generated by the strong electromagnetic waves produced by the use of cellular phone.
To solve this problem, a technique has been disclosed in Japanese Patent Application Publication No. 2005-333180. In this technique, as shown in FIG. 5, a gasket 34 consisting of a conductive cloth or the like having both of elasticity and electric conductivity is put between the base part 10 and the peripheral edge part 20a of the microphone cover 20, whereby the shield provided by the base part 10 and the microphone cover 20 is made further reliable.
Unfortunately, this shield still has a problem as described below. Although the externally threaded screw 22 for fixing the microphone cover 20 is usually made of iron to assure the mechanical strength, the externally threaded screw 22 is painted, for example, in black from the viewpoint of design because the head of the externally threaded screw 22 is exposed to the outside. Also, the punched plate used as the microphone cover 20 is similarly painted, for example, in black.
Therefore, the electrical connection between the externally threaded screw 22 and the microphone casing 1 is unstable in view of high frequency. Also, since the externally threaded screw 22 penetrates the microphone cover 20 and gets into the microphone casing 1, electromagnetic waves may intrude into the microphone casing 1 with the externally threaded screw 22 serving as a magnetic core. Incidentally, it has been confirmed that when the externally threaded screw 22 is removed, the noise caused by electromagnetic waves reduces.